Spin Doctor: Resin Casting the Coils for the Bass Neo Sidewinder

For this pickup, I had to come up with a way to cast a fairly narrow coil in epoxy. I had to do it in such a way so that the magnets would not be covered in resin and the bottom needed to have space for the wire terminals. What I decided to try was to make a two-part silicon mold and cast the coils without the magnets so that I could install them later.

First, I made a two-part mold. This basically involved making one cast using modeling clay as a filler. I used ball bearings as registration points.

I backed one edge all the way to the side of the box. Leaving on side open, this is where I will actually pour the epoxy into the mold. The wire terminals will be sticking out of the back. Once I poured the Smooth-On OOMMOO 30 silicone casting compound and let it set, I got this:

From which I made the other half of the mold – I’m actually making a couple of different parts here…

Once I go the completed mold, I placed the coil into the mold. I included a couple of nubs to hold it in place so that the coil is suspended in the mold while the epoxy flows underneath it. It’s tight enough so that not too much resin goes into the slug holes, but just in case, I added some registered drill marks that would be cast into the piece. This coil hasn’t had the terminals installed yet – I just placed it in there for demonstration.

Next, I pour the West Systems Epoxy. I used black Mixol as a colorant. I ended up mixing a little too much epoxy – the whole cast only used 1/2 teaspoon of material! The fixture is to keep the sides from flexing – otherwise, the epoxy will just splooch out of the sides. Note the li’l terminals sticking out of the top left corner.

And here we go!

Not too bad! First, let me disclose that there was a teeny weeny bit of resin in a couple of the holes – I just got too excited and cleaned them out before I took the picture. The sides might need a little sanding and cleanup, but they won’t be exposed, so I don’t have to worry too much about it. I will have to do some facing on the top part which will be visible. I had the casting in the mold for about 8 hours on a fairly hot and humid day, and the epoxy still seemed a tad soft. I may let it harden for another day before I start sanding the front surface.